1. Field of the Invention
The present invention relates to an electro-photographic color image forming apparatus, such as a copier, a printer or a facsimile device. More particularly, the present invention relates to a color image forming apparatus having a mono color printing function for forming images using only a black color developer, and a method thereof.
2. Description of the Related Art
An electro-photographic color image forming apparatus forms color images on an image receiving medium, such as a paper. The color images are formed by forming electrostatic latent images on a photoconductor, such as a photoconductive belt or an organic photoconductive drum (OPC), developing the electrostatic latent image using developers of predetermined colors and transferring the developed image onto the image receiving medium.
FIG. 1 shows a conventional tandem type electro-photographic color image forming apparatus 1.
Referring to FIG. 1, a paper cassette 11 is disposed at a bottom portion of a main body M of the color image forming apparatus 1. The paper cassette 11 loads a stack of paper S, and a pick-up roller 12 picks up the paper S one by one. The picked-up paper S is conveyed to a regist roller 14.
The regist roller 14 conveys the paper to a conveyer belt 2. The conveyer belt 2 is rotated by a plurality of rotating rollers such as a driving roller 18, a first and a second tension rollers 20 and 21, and a passive roller 19. The conveyer belt 2 conveys the paper in an upward direction. A pressure roller 22 is disposed to face the passive roller 19 to pressurize the conveyer belt 2 to the passive roller 19.
A predetermined bias voltage is supplied to the pressure roller 22. When the pressure roller 22 pressurizes the paper S to the conveyer belt 2, the paper S adheres onto the conveyer belt 2 because of the supplied bias voltage.
As shown in FIG. 1, four photoconductors are vertically disposed to face the conveyer belt 2. That is, a yellow color photoconductor 1y, a magenta color photoconductor 1m, a cyan color photoconductor 1c and a black photoconductor 1k are vertically disposed from the bottom portion to the top portion in the image forming apparatus 1.
Chargers 3y, 3m, 3c and 3k, development units 5y, 5m, 5c and 5k and cleaning blades 6y, 6m, 6c and 6k are disposed around corresponding one of the photoconductors 1y, 1m, 1c and 1k, respectively. Transfer rollers 8y, 8m, 8c and 8k are disposed at an inner side of the conveyer belt 2.
A developer container of each development unit 5y, 5m, 5c or 5k contains a developer of corresponding color. Developer images of four colors are formed on corresponding photoconductors 1y, 1m, 1c and 1k, respectively through a sequence of image forming processes. Those formed developer images of four colors are transferred in order to the paper S while the paper S is being conveyed by the conveyer belt 2.
After forming the developer images on the paper S, the paper S is conveyed to a fuser 15 having a fusing roller 15a and a pressure roller 15b. The fusing roller 15a and the pressure roller 15b fix the developer images onto the paper S, permanently. Then, a discharge roller 16 outputs the paper S to an output tray 17 disposed at a top portion of the main body M.
The conventional color image forming apparatus generally includes functions for a full color printing mode and a mono color printing mode. In the full color printing mode, the conventional color image forming apparatus forms images using developers of yellow y, magenta m, cyan y and black k. On the contrary, the conventional color image forming apparatus forms images using only developer of black k in the mono color printing mode. Accordingly, a user often selects the mono color printing mode to reduce a maintenance cost of the developers and to print documents at high speed.
While the conventional color image forming apparatus is forming images on the paper in the mono color printing mode, developer images of yellow, magenta and cyan are not formed on the photoconductors 1y, 1m and 1c. However, the photoconductors 1y, 1m and 1c must be rotated to avoid the mechanical frictional force generated between the photoconductors 1y, 1m and 1c and the conveyer belt 2 because the transfer rollers 8y, 8m and 8c pressurize the conveyer belt 2 to the photoconductors 1y, 1m and 1c at a predetermined pressure to be in contact with the photoconductors 1y, 1m and 1c. 
Since the photoconductors 1y, 1m and 1c are rotated, the cleaning blades 6y, 6m and 6c sweep the surfaces of the photoconductors 1y, 1m and 1c despite there not being developers applied on the surfaces of the photoconductor 1y, 1m and 1c in the mono color printing mode. If the cleaning blades 6y, 6m and 6c clean the photoconductors 1y, 1m and 1c when there is no remaining developer on the surface of the photoconductors 1y, 1m and 1c, the edges of the cleaning blades 6y, 6m and 6c are worn and damaged, and the surfaces of the photoconductors 1y, 1m and 1c are scratched because there is no developer applied on the surface of the photoconductor to work as a lubricant between the photoconductor and the cleaning blade. These scratched photoconductors 1y, 1m and 1c and the damaged cleaning blades 6y, 6m and 6c result in images of poor quality on the paper. Therefore, the image quality of the conventional image forming apparatus is degraded thereby.
Accordingly, there is a need for an improved color image forming apparatus having a mono color printing function for forming images using only a black color developer that prevents photoconductors and cleaning units from being damaged, and a method thereof.